Parent Company Financial Information, Condensed Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities [Abstract]
Net income	$ 78,409	$ 76,425	$ 75,074
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Stock-based compensation	4,378	4,086	3,521
(Losses) gains on sales of available-for-sale securities	644	(3,087)	(49)
Net change in other liabilities	(10,697)	5,236	5,286
Net change in other assets	364	15,386	(19,601)
Net cash provided by operating activities	109,510	124,543	87,811
Investing activities [Abstract]
Purchases of available-for-sale securities	(597,428)	(481,262)	(197,652)
Sales and maturities of available-for-sale securities	324,781	299,302	236,133
Purchases of premises and equipment	(3,308)	(8,193)	(9,003)
Net cash used in investing activities	(629,960)	(504,004)	(143,741)
Financing activities [Abstract]
Proceeds from the issuance of shares to employee benefit plans and other stock plans	3,700	7,649	6,256
Purchases of treasury shares	(17,193)	(26,797)	(72)
Cash dividends and payments for fractional shares	(38,880)	(38,149)	(36,905)
Net cash provided by financing activities	529,334	373,122	43,640
Net increase (decrease) in cash and cash equivalents	8,884	(6,339)	(12,290)
Cash and cash equivalents at beginning of year	140,297	146,636	158,926
Cash and cash equivalents at end of year	149,181	140,297	146,636
NBT Bancorp Inc [Member]
Operating activities [Abstract]
Net income	78,409	76,425	75,074
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Stock-based compensation	4,378	4,086	3,521
(Losses) gains on sales of available-for-sale securities	(652)	(3,034)	49
Equity in undistributed income of subsidiaries	(79,193)	(74,916)	(75,824)
Cash dividend from subsidiaries	10,200	78,200	35,400
Net change in other liabilities	(8,596)	6,770	18,594
Net change in other assets	22,372	(6,944)	(25,151)
Net cash provided by operating activities	26,918	80,587	31,663
Investing activities [Abstract]
Purchases of available-for-sale securities	(580)	(3,083)	(597)
Sales and maturities of available-for-sale securities	1,783	5,297	140
Purchases of premises and equipment	(278)	(408)	(640)
Net cash used in investing activities	925	1,806	(1,097)
Financing activities [Abstract]
Proceeds from the issuance of shares to employee benefit plans and other stock plans	2,645	7,692	5,943
Purchases of treasury shares	(17,193)	(26,797)	(72)
Cash dividends and payments for fractional shares	(38,880)	(38,149)	(36,905)
Net tax benefit (expense) from stock-based compensation	1,055	(43)	313
Net cash provided by financing activities	(52,373)	(57,297)	(30,721)
Net increase (decrease) in cash and cash equivalents	(24,530)	25,096	(155)
Cash and cash equivalents at beginning of year	28,682	3,586	3,741
Cash and cash equivalents at end of year	$ 4,152	$ 28,682	$ 3,586